This invention relates to drilling boreholes.
The drilling of boreholes is generally carried out using a rotary drilling process. The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these necessities are accomplished by the circulation of a fluid down through the drill string and up to the surface between the drill string and the wall of the borehole.
As the drilling proceeds through the various formations being traversed by the borehole, there are varying degrees of problems encountered because of the porosity of the formations and instability of shale that may be encountered. Drilling fluid density is typically controlled so as to provide sufficient fluid pressure to prevent inflow of fluids from the formation. The pressure must, however, be maintained low enough not to cause harmful rupture of the formation. Nonetheless, there is generally loss of fluid from the drilling fluid which normally results in the laying down of a filter cake of solids from the drilling fluid onto the borehole walls. It is desirable to keep this fluid loss low and thus lay down a relatively thin filter cake. Excessive fluid loss results in waste of drilling fluid, contamination of the formation and the production of a thick filter cake that may adversely affect the integrity of any subsequent cementing operation to secure a casing or liner.
Also, there is a chemical effect which is of equal importance in determining the effectiveness of the drilling fluid to perform its required functions without damaging the formation due to excessive mud weight pore pressure communication.
Peterson, U.S. Pat. No. 4,780,220 (Oct. 25, 1988) discloses glycerine, polyglycerine, or mixtures thereof in drilling fluids to serve as a lubricant, reduce filtrate, control flow properties and minimize sloughing shale problems. Cannon, U.S. Pat. No. 2,191,132 (Feb. 20, 1940) discloses a polyhydroxy alcohol or saccharose to drilling fluid to minimize shale disintegration. Enright et al, U.S. Pat. No. 5,007,489 (Apr. 16, 1991), discloses a drilling fluid having a water-insoluble polyglycol and a surfactant composition to control bit balling.